pre-processor strips out __attribute__((__packed_)_), is this expected?

[Daniel Dickman <didickman at gmail dot com>]

Hello,

Is there a reason why the pre-processor strips out attributes when
-U__GNUC__ is specified and there is at least 1 #include directive?
This seems like surprising behaviour to me. Here's an example:

    $ cat foo.c
    #include <limits.h>

    struct S {
           int a;
    } __attribute__((__packed__));
    $ egcc -E -U__GNUC__ foo.c | tail -3
    struct S {
           int a;
    } ;

But if I remove the #include directive (or if I drop -U__GNUC__) then
attributes don't get stripped by the pre-processor which is what I
expect to happen.

    $ cat foo2.c
   struct S {
           int a;
    } __attribute__((__packed__));
    $ egcc -U__GNUC__ -E foo2.c
    # 1 "foo2.c"
    # 1 "<built-in>"
    # 1 "<command-line>"
    # 1 "foo2.c"

    struct S {
            int a;
    } __attribute__((__packed__));

I tested gcc 4.2.1 and 4.9.2 on OpenBSD/i386 as well as gcc 4.6.3 on
Linux/x86_64 and they all show the same thing. Any clues would be
appreciated.


gcc (GCC) 4.2.1 20070719
Copyright (C) 2007 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

egcc (GCC) 4.9.2
Copyright (C) 2014 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

gcc (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3
Copyright (C) 2011 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.